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Somatic Nervous System and Somatic pain

Chủ Đề: Somatic Nervous System and Somatic pain

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►Ngày Gửi: 23-10-11

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The somatic nervous system

The somatic nervous system (SNS) is the part of the peripheral nervous system that is responsible for carrying motor and sensory information both to and from the central nervous system. This system is made up of nerves that connect to the skin, sensory organs and all skeletal muscles. The system is responsible for nearly all voluntary muscle movements as well as for processing sensory information that arrives via external stimuli including hearing, touch and sight.The SNS consists of efferent nerves responsible for stimulating muscle contraction, including all the non sensory neurons connected with skeletal muscles and skin.

Parts of the Somatic Nervous System:

The somatic nervous system derives its name from the Greek word soma, which means "body." The somatic nervous system contains two major types of neurons: sensory neurons (or afferent neurons) that carry information from the nerves to the central nervous system, and motor neurons (or efferent neurons) that carry information from the brain and spinal cord to muscle fibers throughout the body.
The neurons of the somatic nervous system project from the central nervous system directly into the muscles and sensory organs. The system is a direct path with synapses to connect nearby neurons. The body of the neuron is located in the CNS, and the axon then projects and terminates in the skin, sense organs or muscles. Electrochemical impulses then travel along the axon to the brain or spinal cord.Nerve signal transmission:
The somatic nervous system controls all voluntary muscular systems within the body, with the exception of reflex arcs.
The basic route of nerve signals within the efferent somatic nervous system involves a sequence that begins in the upper cell bodies of motor neurons (upper motor neurons) within the precentral gyrus (which approximates the primary motor cortex). Stimuli from the precentral gyrus are transmitted from upper motor neurons and down the corticospinal tract, via axons to control skeletal (voluntary) muscles. These stimuli are conveyed from upper motor neurons through the ventral horn of the spinal cord, and across synapses to be received by the sensory receptors of alpha motor neurons (large lower motor neurons) of the brainstem and spinal cord.
Upper motor neurons release a neurotransmitter, acetylcholine, from their axon terminal knobs, which are received by nicotinic receptors of the alpha motor neurons. In turn, alpha motor neurons relay the stimuli received down their axons via the ventral root of the spinal cord. These signals then proceed to the neuromuscular junctions of skeletal muscles.
From there, acetylcholine is released from the axon terminal knobs of alpha motor neurons and received by postsynaptic receptors (Nicotinic acetylcholine receptors) of muscles, thereby relaying the stimulus to contract muscle fibers.Reflex arcs

A reflex arc is a neural circuit that creates a more or less automatic link between a sensory input and a specific motor output. Reflex circuits vary in complexity—the simplest spinal reflexes are mediated by a three-element chain, beginning with sensory neurons which activate interneurons in the spinal cord, which then activate motor neurons. Some reflex responses, such as withdrawing the hand after touching a hot surface, are protective, but others, such as the patellar reflex "knee jerk" activated by tapping the patellar tendon, contribute to ordinary behavior.

Somatic pain:

Somatic pain is a type of pain arising when receptors called nociceptors, located in areas like the bones, muscles, and skin, send pain signals to the brain. This pain is often differentiated from what is called visceral pain, another form of nociceptive pain, where nociceptors in the organs are engaged. Visceral and somatic pain are the two main types of nociceptive pain, classified by where they occur on or in the body.

There are clearly many areas of the body where somatic pain can occur. Any cut of the skin can cause it. Exercise too much and cramping muscles may result. Break a bone and the pain resulting from it is somatic pain.